The present invention relates generally to vertebral fixation or defect devices, and more particularly, to an intervertebral defect device for insertion into an intervertebral space using minimally invasive surgical techniques.
As shown in prior art FIGS. 1A-4, it is known in the prior art that the spine 120, also known as the spinal column or vertebral column, supports the upper body, allows head, neck, and trunk motion, and includes twenty-four moveable vertebrae 100 including seven cervical vertebrae 102, twelve thoracic vertebrae 104, and five lumbar vertebrae 106, which extend from the skull to the sacrum. In FIGS. 1A and 3, the directional arrow 101a is pointing in the posterior direction and the directional arrow 101b is pointing in the anterior direction.
Referring to FIG. 3, with the exception of the first, uppermost cervical vertebra 102, each vertebra 100 has a vertebral body 103, a lamina 110, a spinous process 105, as well as facet structures 114 (which form facet joints), two transverse processes 116, and two pedicles 118, one on each side. Each individual vertebra 100 has a large foramen 111, which forms the spinal canal 128 (FIG. 2) when the vertebrae 100 are in their normal anatomical position forming the spine 120. The spinal cord and major nerve fiber groups pass through and are protected by the spinal canal 128. A strong fibrous membrane, the dura mater (not shown), also known as the dura, surrounds the spinal cord, nerve fibers, and fluid in the spinal canal 128.
Referring to FIG. 4, each pair of adjacent vertebrae 100 along with interconnecting soft tissues and an intervertebral disk 121 constitutes a motion segment 122, also known as a functional spinal unit. The combined motions of many such motion segments 122 constitute overall spinal motion at any one time. The intervertebral disk 121 resides in the space between adjacent vertebral bodies, the intervertebral space 130, also known as the interbody space or disk space. The level of each particular intervertebral space 130 and intervertebral disk 121 is identified by naming the vertebrae 100 superior and inferior to it, for example LIV-V in the case of the intervertebral space 130 and intervertebral disk 121 between the fourth and fifth lumbar vertebrae LIV, LV.
Referring to FIGS. 2 and 4, the superior surface 100a and the inferior surface 100b (FIG. 2) of each the lumbar and thoracic vertebral bodies 104, 106 are concave (the shape of the vertebral space 130 shown in phantom in FIG. 4). Owing to the shapes of the inferior and superior surfaces 100a, 100b of the vertebral bodies 104, 106, the lumbar and thoracic intervertebral spaces 130 and intervertebral disks 121 are biconvex.
Situations arise in which one or more motion segments 122 do not have adequate support or stability, which can lead to pain, deformity, stenosis of spinal canal or neuroforamina, and impairment or loss of nerve function. In some cases, surgical spine fusion is considered. Spine fusion is a process of growing bone between two or more adjacent vertebrae 100 such that the adjacent vertebrae 100 of a motion segment 122 will move only in unison. This process involves placing bone, or material to guide or stimulate bone growth, in proximity to exposed bone of the vertebrae 100, and then allowing time for new bone to grow and form a structurally strong connection, or fusion, between the adjacent vertebrae 100. The earliest such procedures took place approximately a century ago, and the procedures have developed over many years, including various attempts to fuse posterior structures of the spine 120 such as the spinous process 105, lamina 110, facet joints 114, and transverse processes 116.
Recently, there has been more interest in fusion involving bone growth directly between adjacent vertebral bodies 100 within the intervertebral spaces 130. Large amounts of well vascularized bone are in close proximity, there is a large surface area available, and the inherent compression force applied between vertebral bodies 100 by muscle tension and the upright position of the human body enhances bone formation and strength. The intervertebral disk space 130 has therefore become a major focus in interbody fusion surgery. During such surgery, the disk space 130 is cleared as much as possible, and cartilage and abnormal surface bone, also known as endplate bone, from adjacent vertebral bodies 100 is removed, after which material is placed in the space to promote fusion. However, loose bone fragments do not provide structural support and therefore fusion is often unsuccessful. Structural bone grafts from the patient or donors have been successful, but may give rise to pain and complications if from the patient, and the additional risk of disease transmission if from a donor.
Vertebral defect devices are increasingly used to assist with fusion between vertebral bodies. Such devices need sufficient strength to provide support to prevent excessive collapse of the vertebral space 130 between vertebrae 100 which could result in stenosis of the spinal canal or neuroforamina, progressive deformity, impairment or loss of nerve function, or pain. Such devices also preferably provide at least one compartment to fill with bone, or material which assists in bone growth, in order to maintain close contact with vertebral bone as new bone is encouraged to bridge across the space 130 involved.
Referring to FIG. 3, which shows a single plan view of a vertebra 100, it is known in the art that vertebral defect devices can be inserted from several directions (indicated by arrowed lines). Typically, vertebral defect devices are inserted through anterior or lateral approaches E, F. Anterior approaches E allow more access to the disk space, but require more destruction of the annulus. In the lumbar spine 106, bilateral placement of devices has sometimes been necessary to achieve adequate stability.
Lumbar surgery is experiencing an evolution to minimally invasive surgery by accessing the intervertebral disk space 130 through a posterior approach A, B, C, D. This trend has led to the need for devices that can be inserted through small portals or working channels, usually through one small incision from the posterior direction 101a laterally spaced from spine 120. In lumbar fusion, vertebral devices should assist with rapid fixation to minimize the need for extensive additional fixation with bone screws, rods, and plates. Surgeons generally prefer posterior approaches for lumbar spine procedures due to the morbidity of anterior approaches, which also cause adhesion of major vessels and make repeat anterior surgery very dangerous and even life threatening. When posterior lumbar fusion is performed, there is an opportunity to approach the spine 120 through the neuroforamina 124 (FIG. 4) and insert the vertebral defect devices through a small space free of vital structures which is located lateral to the dura in the spinal canal 128, medial to the large nerve root passing through the neuroforamina 124, and bordered inferiorly or caudally by the pedicle 118 of the vertebra 100 below the involved disk space 130. The distance between peripheral edges of the vertebrae 100 at the disk space 130 is often small so that entry of devices has required considerable bone removal to safely impact devices into the disk space 130. If a device with a distal end of 3 mm or larger in height or transverse dimension is impacted into the disk space 130, it will frequently displace medially or laterally, and involve nerve structures. When working through a small portal or working channel, it is difficult to see this displacement, which makes some devices dangerous in this regard.
Conventional vertebral devices adapted for bilateral surgery do not lend themselves to being used in minimally invasive surgery. When inserting a conventional vertebral device, a distraction tool is used to separate adjacent vertebrae 100 and open the disk space 130 on one side of the spine 120 to allow insertion of the vertebral device on the other side of the spine 120. Alternatively, pedicle screws on one or both sides of the spine 120 may be used with a distraction instrument to spread open the disc space 130 for insertion of one or more vertebral devices. Cylindrically-shaped devices, inserted through posterior and transforaminal approaches A, B, C, D are associated with increased potential for nerve root or dural injury, particularly when drill tubes and reamers are used to prepare the disk space 130 for fusion.
What is needed is a vertebral defect device used in minimally invasive surgery that is designed to achieve rapid fixation while preventing excessive subsidence. The device should reduce the potential for neural injury during insertion, and reduce or eliminate the need for bilateral lumbar pedicle screws. The device should have excellent support strength, allow for the insertion of fusion material within the device and allow for viewing of the bone growth. The device is preferably placed through an incision and directly into the disk space without manual or mechanical separation of the vertebrae.